Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

22Textbook of Visual Science and Clinical Optometry

ii.Processes of the amacrine cells

iii.Fibres of Müller

iv.Branches of the retinal arteries and veins.

h.Ganglion cell layer: Ganglion cells are multipolar nerve cells with clear oval nucleus and well-developed Nissl granules. Ganglion cells form the second order of neurons. Their axons pass into the nerve fibre layer and arborize on cells in the lateral geniculate body and superior colliculus (central nervous system).

i.Nerve fibre layer: It consists of;

i.Axons of the ganglion cells which pass through lamina cribrosa and form the optic nerve.

ii.Fibres of Müller

iii.Retinal vessels.

The bundles of axons of ganglion cells run parallel to the surface of the retina and converge towards the optic disc. They are nonmyelinated. Nasal fibres reach optic disc directly. The temporal fibres pass above and below the macula to reach the optic disc (Fig. 1-11). The macular fibres directly reach the temporal side of the optic disc and form the papillomacular bundle (Fig. 1-11).

j.Inner limiting membrane: It is a thin hyaloid membrane separating the retina from the vitreous. The feet of the fibres of Müller are attached to it.

Fig. 1-11: Direction of nerve fibres reaching optic disc

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Retina is topographically divided into central 4.5 mm diameter zone (fovea centralis being the centre) called posterior pole, peripheral retina and intermediate equatorial retina.

BLOOD SUPPLY OF RETINA

a.Retinal pigment epithelium, layers of the rods and cones, outer limiting membrane and outer nuclear layer are avascular layer and are nourished by the choriocapillaries.

b.Outer plexiform layer is supplied by both the choriocapillaries and the retinal vessels.

c.Rest of the retinal layers are supplied by the retinal vessels.

d.Macula lutea and fovea is exclusively nourished by the choriocapillaries. Macula is occasionally supplied by a cilioretinal artery which emerges from the temporal margin of the optic disc. Cilioretinal vein is extremely rare.

The endothelial cells present in the capillaries of the retina are closely adherent to each other by tight junctions and prevent free flow of fluids and other solutes between the capillaries and the retinal tissue and form the blood-retinal barrier.

OPTIC NERVE

•Optic nerve consists of approximately 1 million axons of the ganglion cells of retina, glial cells and the meningeal sheaths.

•It extends from the optic disc to the optic chiasma.

•It can be divided into 4 portions depending on it’s location;

i.Intraocular— It extends from the optic disc to just posterior to the lamina cribrosa

ii.Orbital

iii.Intracanalicular— The portion within the optic foramen

iv.Intracranial.

The optic disc is vertically oval, 1.5 mm in diameter. It is strikingly paler in colour than the surrounding retina. It is located 3 mm nasally and slightly at a upper level than the fovea. Physiological cup, the funnel shaped depression within the optic disc is lighter in colour than the peripheral neuroretinal rim. The nerve fibres are transparent within the retina and at the optic disc since, they are nonmyelinated. Just behind the lamina cribrosa they

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STRUCTURE

Lens consist of (Fig. 1-12) the following:

Lens Capsule

It is acellular and envelops the lens completely. It is a basement membrane of the lens epithelium. It is thinnest at the posterior pole. Only the anterior capsule is lined by the single layer of epithelium.

Lens Epithelium

It consists of a single layer of cuboidal cells present in the anterior lens capsule (A – cells) and in the equatorial bow region (E – cells). The A – cells (LEC) present in the anterior capsule are not directly involved in the formation of new lens fibres. The equatorial bow/ E – cells show mitotic activity to form new lens fibres. As new cells are formed, these lens fibres elongate and lose their nuclei. Older fibres are continuously pushed centrally.

Fig. 1-12: Structure of the crystaline lens

Anatomy of the Eyeball 27

Lens Substance/Material

It consists of the cortex, epinucleus, nucleus and sutures. The most externally located lens fibres which lie beneath the lens capsule form the cortex.

The fibres of central embryonic nucleus meet at the sutures shaped Y. The anterior Y suture is erect, whereas the posterior suture is inverted Y. As development proceeds successive layers of nucleus are formed externally and added to the central embryonic nucleus, viz. foetal, infantile and adult nucleus.

VITREOUS HUMOUR

It is a clear, transparent, colourless jelly that fills the posterior 4/ 5th of the eye, i.e. the space behind the lens and the zonule of Zinn. Volume of vitreous is 4.5 ml (approx.). Anteriorly vitreous has a saucer like depression called fossa patellaris to lodge the lens. Through it’s centre runs hyaloid canal, which is remnant of the hyaloid artery. Vitreous humour is loosely adherent to the retina. However, it is firmly attached to the;

i.Margin of the optic disc (Area Martegiani)

ii.Pars plana of the ciliary body near ora serrata. 1.5 mm broad zone of the ciliary epithelium next to ora serrata (termed vitreous base).

iii.Macula

iv.Central 9 mm diameter zone of the posterior capsule of the lens (ligamentum hyaloideo-capsulare of Weiger, (Fig. 1-13)).

Fig. 1-13: Anatomical areas of the vitreous chamber

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From developmental point, vitreous is divided into 3 parts:

i.Primary vitreous— From the posterior pole of the lens to the optic disc is a space called Cloquet’s/Stilling’s/hyaloid canal (Fig. 1-13) containing remnants of the primary vitreous.

ii.Secondary vitreous— It fills the vitreous chamber.

iii.Tertiary vitreous— The zonules of Zinn represent tertiary vitreous.

BLOOD SUPPLY OF EYEBALL

ARTERIAL SUPPLY

The eyeball is supplied by the branches of the ophthalmic artery, which is a branch of the internal carotid artery. Ophthalmic artery enters orbit through the optic foramen and divides into two sets of vascular system. The retinal vasculature, i.e. central retinal artery supplies inner half of the retina, whereas the ciliary system formed by other branches supply the uveal tract, the outer half of the retina and the optic nerve. The branches of ophthalmic artery are:

i.Central retinal artery: It enters the optic nerve 15–20 mm behind the eyeball. It divides at or just posterior to the surface of the optic disc into two main trunks superior and inferior. They again subdivide into superior and inferior temporal and nasal arterioles which are located within the nerve fibre layer of the retina. These retinal arterioles are end arteries and each supply a quadrant of the retina. The capillaries on the surface of the optic disc are derived from these retinal arterioles.

ii.Short posterior ciliary arteries (20 in number)— They pierce the sclera in the ring around the optic nerve and form the circle of Zinn. They enter the uveal tract and divide into smaller vesselstoformthevesselsofthechoroidandchoriocapillaries.

iii.Long posterior ciliary arteries (2 in number)— They pierce the sclera on either side of the optic nerve and pass through the suprachoroidal space in horizontal location to reach ciliary body. Now, each artery divides into two branches and the two branches again anastomose with each other and anterior ciliary arteries, at the base of the iris to form “major arterial circle of iris” (Fig. 1-14).

Anatomy of the Eyeball 29

iv.Anterior ciliary arteries (7 in number)— They are the terminal branches of muscular branches of the ophthalmic arteries. They enter the eyeball through rectus muscles (two for each recti, except lateral rectus which has only one). They pierce the sclera 5 to 6.5 mm from the limbus and contribute to the formation of ‘‘major arterial circle of iris’’.

v.Recurrent meningeal artery

vi.Lacrimal artery

vii.Numerous recurrent arteries.

Twigs from the major arterial circle run radially into the iris and form a circular plexus of anastomosis at the collarette near the pupillary margin, called “minor arterial circle of iris” (Fig. 1-14).

Fig. 1-14: Vascular supply of the eyeball

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VENOUS DRAINAGE

It occurs through:

i.Vortex veins— Majority of venous drainage occurs through the tributaries of 4 vortex veins, which enter the sclera just behind the equator and pass obliquely through it. They drain into ophthalmic veins.

ii.Anterior ciliary veins – They drain blood from the outer part of the ciliary muscle and the ciliary body via ciliary venous plexus.

iii.Short posterior ciliary veins – They drain blood away from the sclera.

iv.The retinal veins – The 4 tributaries that correspond roughly to the branches of the central retinal artery unite at or just behind the optic disc to form the central retinal vein. The central retinal vein usually drains into cavernous sinus, after

giving a branch to the superior ophthalmic vein. It may drain into the superior ophthalmic vein occasionally.

The major branches of the central retinal artery and tributaries of central retinal vein are located within the nerve fibre layer of the retina. In most of the retinal area two groups of capillary network exist. The superficial one is located within the nerve fibre layer, whereas the deep one is located between the outer plexiform layer and inner nuclear layer.

It is easy to distinguish the retinal arteries from the veins. The arteries are narrower (3/5th of the veins), bright red in colour with a well-defined light streak along their lumen. The veins are wider and less bright in colour. Retinal venous pulsation is physiological and is seen in significant number of people. Retinal arterial pulsation is pathological and is seen in glaucoma, aortic incompetence, etc.